According to the Kraftfahrttechnisches Taschenbuch, Robert Bosch GmbH, 20th edition, page 502 ff., the term “braking equipment” is understood to mean all brake systems of a motor vehicle for the purpose of reducing its speed, bringing it to a standstill as well as holding it at a standstill. Braking equipment generally encompasses a service brake, a holding brake and an auxiliary braking means.
The service braking means makes it possible to reduce the speed of the motor vehicle during its normal traveling operation or to bring the motor vehicle to a standstill. The service braking means is usually controlled by a vehicle driver either manually and/or by foot by means of a suitable actuating mechanism (e.g. brake pedal) and/or by a vehicle control for controlling the traveling operation (“cruise control”, “Tempomat”).
The “holding braking means” is usually embodied separately from the service braking means and makes it possible to predetermine and maintain braking forces for the motor vehicle. The holding braking means is normally used to safely hold the motor vehicle at a standstill even in the absence of the vehicle driver. The holding braking means is usually actuated by the vehicle driver either manually and/or by foot in order to predetermine a desired braking force. Actuating mechanisms for holding braking means are generally designed such that a predetermined braking force is maintained until the vehicle driver takes appropriate measures such as, e.g., again actuating the respective actuating mechanism (“releasing a hand brake”).
The “auxiliary braking means” generally serves to partly or completely provide the function of the service braking means should the latter malfunction. Means of the service and/or holding braking means are usually also used for the auxiliary braking means. It is thus possible, for example, that one brake circuit of a service braking means with a plurality of brake circuits or the holding braking means takes over the function of the auxiliary braking means.
Various solutions are known to increase the ease of operation of a motor vehicle for operating states, in which the braking equipment is used (in part), and to increase the safety obtained in the operation of braking equipment during traveling and standstill phases of a vehicle.
For example, so-called “braking assistants”, which assist a vehicle driver when actuating a service braking means, are used in vehicles. Especially in the case of operating states of a vehicle, in which high braking forces are required in brief periods, the braking assistant controls the service braking means in dependence of a current operating state of the vehicle so as to increase the provided braking force in comparison with a braking force, which would be produced just by the vehicle driver without using the braking assistant.
From DE-OS-24 20 252 it is known to activate and de-activate the holding brake of a motor vehicle in dependence of a position of an accelerator pedal. In this case, the holding brake is activated to produce a fixedly predetermined braking force when the accelerator pedal is not actuated. The holding brake is accordingly de-activated, whereby the fixedly predetermined braking force is reduced as soon as the accelerator pedal is actuated.
According to DE 199 09 326 A1, and corresponding U.S. Pat. No. 6,554,744, both of which are incorporated by reference herein, an accelerator pedal having a rest position and a neutral position is used to control a braking force generator of a motor vehicle. The accelerator pedal can be moved from its rest position into its neutral position, from which a further movement results in an increase in the driving torque and engine performance, respectively, of the motor vehicle engine. Positions of the accelerator pedal between the rest position and the neutral position are not used to control the motor vehicle. In order to brake the motor vehicle during traveling operation without actuating a brake pedal, the braking force generator builds up a previously specified braking force when the accelerator pedal reaches its rest position. The braking force generator accordingly reduces the specified braking force in controlled manner when the accelerator pedal leaves its rest position in order to accelerate the motor vehicle again. The motor vehicle may alternatively be braked by determining a movement of the accelerator pedal from the neutral position into the rest position and using the braking force generator to build up the previously specified braking force according to the determined accelerator pedal movement. In this case, if the motor vehicle is accelerated again, a movement of the accelerator pedal from the rest position into the neutral position is determined and the braking force generator is controlled to reduce the previously specified braking force according to the accelerator pedal movement.
A starting assistance for a motor vehicle on a slope and a method for controlling the starting assistance is known from DE 36 18 532 A1, and corresponding U.S. Pat. No. 4,717,207, both of which are incorporated by reference herein. Here, a brake system comprises, disposed between the master brake cylinder and the wheel brake cylinders, a valve, which is closed or opened in order to maintain or release the brake actuating pressure. The brake actuating pressure produced by the vehicle driver by means of the brake pedal is maintained when the motor vehicle is stopped and, especially, when it is stopped on a slope. For this purpose, the valve of the brake system is closed when it is ascertained, on the basis of parameters, which characterize the actual operating state of the motor vehicle, that the motor vehicle stands. In order to determine the standstill of the motor vehicle, a check is carried out as to whether the actual traveling speed is zero, whether the clutch is released, whether the first gear or the reverse gear is engaged and whether the angle of inclination of the motor vehicle corresponds to or is greater than a predetermined angle of inclination. If these conditions are fulfilled, the valve is closed and the vehicle driver can release the brake pedal without the motor vehicle then moving as the braking force required for standstill is maintained by means of the valve. The brake actuating pressure can be increased by the vehicle driver actuating the brake pedal again, in which case the valve again serves to maintain the re-set brake actuating pressure.
When the motor vehicle is brought into a traveling state from standstill, the instantaneous torque, which is produced by the vehicle's engine, is detected and compared with an engine torque, which is necessary for starting the stopping motor vehicle. Here, the engine torque required for starting may be a predetermined nominal torque of the engine, which is corrected with regard to the angle of inclination and in dependence of the weight of the motor vehicle. If the instantaneous engine torque corresponds to or is greater than the engine torque required for starting, the valve is opened, whereby the previously maintained brake actuating pressure is reduced.
A conventional torque sensor on the engine, a torque sensor along the drive train from the engine to the wheels, a sensor associated to the clutch or an arrangement comprising two switches disposed on a rotatable part may be used to determine the engine torque. The two switches, which may be disposed, for example, on brake shoes of the wheel brakes, detect a rotation of the rotatable part as well as the direction thereof in order to determine the actual engine torque from these.
If, when the motor vehicle is in the state of standstill described above, it is determined that the holding brake of the vehicle has been actuated by the vehicle driver, i.e. is applied, the valve of the brake system is not closed. If the vehicle driver no longer actuates the brake pedal when the motor vehicle is in this state, the brake actuating pressure therefore drops rather than being maintained by the valve.
The devices described above have disadvantages, which is why they are in particular unsuitable for use in a “stop-and-go” operation or do not represent any assistance for a vehicle driver.
Thus, braking assistants serve to assist a vehicle driver in the generation of a braking force, in particular when emergency braking. As described above, braking assistants are activated upon recognition of an operating state of the vehicle brake system for emergency braking. Operating states of this kind do not normally occur in a “stop-and-go” operation, which is why braking assistants are not activated in this case.
According to DE-OS-24 20 252, the braking force of the holding brake is provided abruptly when the accelerator pedal is in an unloaded position. Accordingly, the holding brake is released abruptly, i.e. the braking force is removed abruptly, when the accelerator pedal is loaded. In this case, a fixedly predetermined braking force, which is not adapted to the current operating state of the motor vehicle, is provided by the holding brake in its actuated state. This means that, for example, the fixedly predetermined braking force may be insufficient for holding the motor vehicle at a standstill when it is on a slope. Moreover, the holding brake is always activated when the accelerator pedal is in an unloaded position. The holding brake is consequently also activated during traveling operation of the motor vehicle, so that unwanted or uncontrollable traveling states may occur, especially at high speeds. This type of actuation of the holding brake also increases wear as the holding brake does not, as intended, just provide braking forces when the motor vehicle is at a standstill.
According to DE 199 09 326 A1, and corresponding U.S. Pat. No. 6,554,744, both of which are incorporated by reference herein, the brake system provides braking force in dependence of positions or movements of the accelerator pedal. The purpose of this is to control the traveling speed of the motor vehicle without additional actuation of the brake pedal in traveling states. Therefore, there it is not possible to let the motor vehicle roll when the accelerator pedal is not actuated, which is desirable in particular where a “stop-and-go” operation is concerned, as braking forces are always automatically produced in this case. Furthermore, any actuation of the brake pedal de-activates the control of the brake system via the accelerator pedal. The brake system described therein consequently behaves like a conventional brake system when the vehicle is brought to a standstill by actuating the brake pedal.
The starting assistance according to DE 36 18 532 A1, and corresponding U.S. Pat. No. 4,717,207, both of which are incorporated by reference herein, only automatically provides braking forces if the vehicle is already at a standstill. As long as the vehicle is still moving, the brake system described therein operates in a conventional manner and consequently does not assist the vehicle driver in a “stop-and-go” operation.